This invention relates to a combination of strip and auger mining of thin seam hydrocarbonaceous deposits, particularly coal.
Coal which is man's most abundant source of energy, has been mined in the U.S. for about three hundred years. Deposits are known to exist in almost every state of the Union with the largest deposits of high quality coal such as bituminous and anthracite, being located east of the Mississippi River with particularly large deposits in such states as Pennsylvania, Ohio, West Virginia, Kentucky and Illinois. Coal exists in seams which vary in thickness from a matter of inches to hundreds of feet with an average thickness in the United States of about 5.5 feet. Most of the coal mined is found in seams which vary in thickness from 3 to 10 feet and which are located as surface outcroppings to depths of several thousand feet. The estimated recoverable coal reserves are on the order of 150 billion tons with annual 1983 production estimated at about 900 million tons.
Up to the start of the 20th century, most of the coal mined came from the eastern states which coincided with the largest concentrations of people and industry. In the eastern states a substantial portion of the coal lies in seams 100 or more feet underground. At that time essentially all of the coal was underground mined with access to the coal seam by adit entry (drift mining) or by a sloped or vertical shaft depending upon geological conditions. The method of extracting the coal is now almost universally mechanized in the traditional room and pillar method. Where surface conditions permit collapse or subsidence of the overburden, longwall mining is becoming more common, particularly with the development of new mining equipment. The same effect may be had by retreat mining, or removing the supports in a room and pillar mine when backing out of a fully developed mine or panel. Of more recent vintage is the use of auger mining in an underground mine. In this method a drill screw followed by scroll sections are power driven into the exposed face of a coal seam with a cylinder of coal removed. When combined with underground mining, auger mining is generally used only around the periphery of a working panel where a shallow overburden may cause squeezing or where there are bad roof conditions. As will be described below, auger mining is more typically used in conjunction with strip mining.
The advent of strip mining occurred during the later part of the 1800's and its growth generally paralleled the development of large earth moving equipment. The basic types of equipment used in strip mining are the power shovel and the dragline. From rather modest beginnings, shovels are now produced with dippers having a capacity in excess of 150 cubic yards and a dipper stick reach or working radius of up to 200 feet. Draglines are now produced with buckets up to 200 cubic yards with a boom of more than 400 feet and a working radius of over 300 feet. The principle operating difference between a shovel and a dragline is that a shovel will dig only at grade or above while a dragline will also dig as much as 185 feet below grade. The choice is often dictated by topography or the type of strip mining practiced. Bucket wheel excavators may also be used for strip mining but so far have found limited use in the United States.
The essentials of strip mining are to remove the earth overburden over a coal seam thereby exposing the bed so that it can be extracted in its entirety. There are two basic methods of strip mining--contour mining and open cast or area mining.
Contour mining is used in hilly or mountainous country where the coal seam outcrops on the side of the hill. The top of the coal seam is exposed by removing the overburden to ever increasing depths into the hill and dumping the overburden waste on the downslope of the hill or in valley fills. The overburden cut into the hill will proceed until the capacity of the excavating machine is reached or until the overburden can no longer be economically removed. The economics, of course, depend on the stripping ratio or the ratio of overburden thickness to the seam thickness. Seldom are stripping ratios in excess of 40 economically feasible. The effective or average stripping ratio may be reduced in those situations where more than one coal seam outcrops and more than one seam may be mined in successive stripping operations. In contour mining however, the height of the highwall is generally the controlling factor with about 150 feet being the practical limit.
Once the practical limit of contour mining is reached, coal recovery may be greatly improved by auger mining into the exposed seam at the base of the highwall. Augers with a diameter approximately equal to the seam thickness, up to about 48 inches, are power driven into the seam to depths of over 200 feet. Auger holes are generally spaced along the seam face to leave an unmined section or rib between holes sufficient to support the overburden for at least the short term. Typically about 40% to 60% of the exposed coal may be removed by this means thereby having the effect of improving the stripping efficiency.
Open cast or area mining is used in flat or gently rolling country which is underlain by a coal bed. A box cut or furrow of overburden is removed to expose the top surface of the coal seam where the coal is removed by normal means. A box cut generally progresses in a straight line from boundary to boundary. Each successive cut works into the exposed edge of the proceding cut and proceeds in the opposite direction with the overburden waste being placed in the void left by the previous cut or furrow. The process proceeds just like plowing a field--hence the sometime used name of furrow mining. The size of the box cut is determined by the size of the equipment used. The amount of overburden which can be removed is generally determined by economics which in turn relate to the stripping ratio and/or the coal thickness. For example, it is not practical to mine a seam less than 18 inches thick nor a field with a stripping ratio much over 50 or with an absolute overburden depth of more than 150 feet.
Contour mining, with or without auger mining assist, will recover coal which generally cannot be underground mined. The same may or may not be true of open cast mining. An underground mine generally requires 100 or more feet of overburden, must have a competent roof and should have access to coal seams of 30 inches or more. Open cast strip mining may be ideal at these cutoff limits. There are known to be extensive coal beds in the eastern part of the United States where the seams are too thin to underground mine and stripping ratios are too high to justify strip mining. There are large areas of Illinois, Indiana, Ohio and Kentucky which fall within these limitations.
Although not presently of much commercial significance, many of the open cast mining methods and limitations stated above equally apply to tar sands. There are presently known to be more than 500 separate tar sands deposits in 22 separate states in the United States having estimated recoverable reserves of over 30 billion barrels of extracted hydrocarbons. A substantial portion of these deposits can be open pit mined and as with coal there exists a significant portion which cannot be economically mined because of the high stripping ratio in conjunction with the limited hydrocarbon recovery possible with thin deposit seams.
It is, therefore, highly desirable to provide an improved and more efficient mining method for recovering thin seams of hydrocarbonaceous materials having high stripping ratios.